In the early days of hearing aids, there was no feasible way to acoustically measure the amount of gain being provided by a specific hearing aid at the time the hearing aid was being dispensed in a clinic. Accordingly, audiologists had to rely on the gain settings reported in the manufacturer's specifications when setting the gain of these devices to match a prescribed gain for a given patient. However, audiologists soon realized that there could be a substantial variation between the amount of gain in the manufacturer's specifications and an actual gain provided by the device. These differences were attributed, at least in part, to variability in the manufacturing process and differences in the acoustics and physical shape of the given patent's ear canals.
By the mid 1970's, tabletop acoustic hearing aid verification systems began to appear on the market. These conventional systems used electronic measurement systems to make real-time measurements of gain as a function of input sound level and frequency for a hearing aid placed in a small sound chamber with an acoustic coupler, as defined by ANSI S3.22 (Specification of hearing aid characteristics). While these conventional systems allowed audiologists to verify, and at least partially, and correct for gain variations due to manufacturing differences, audiologists could not account for variations in performance related to differences in the anatomy of the patient's ear canals.
At about the mid 1980's, real ear measurement (“REM”) hearing aid verification systems began to appear on the market. The REM system was based on the use of probe microphones with tubes that were inserted under the hearing aids. The probe microphones were used to make acoustic measurements of the signal reaching the eardrums of a hearing impaired listener from a remote loudspeaker location in the free field, in accordance with ANSI S3.42 (Testing hearing aids with a broadband noise signal). These REM systems have been conventionally viewed as a preferred method of performing acoustic verification of hearing aids on hearing impaired patients.
While the REM system continues to improve, there remains deficiencies in that the conventional REM system cannot suitably verify the performance of newer generation of hearing aids that incorporate non-linear processing algorithms that shift the frequency composition of an acoustic signal in real time. For example, the newest generation of hearing aids incorporates non-linear, frequency lowering techniques that shift or compress the high-frequency components of sound (that would otherwise be inaudible for listeners having high-frequency hearing loss) down into a lower frequency range where the signal may be audible for these listeners. However, a serious impediment to the proper implementation of the technology is that current tabletop and real-ear acoustic verification systems for hearing aids do not provide a convenient way to evaluate or visualize the effect frequency lowering has on the signal produced by a hearing aid. Frequency lowering algorithms are now implemented on a significant proportion of the hearing aids dispensed in the US; however, there is presently no system that addresses the issue of acoustic verification in frequency shifting hearing instruments. This poses a significant problem for hearing aid dispensers in the US and world-wide. Since REM is considered the current standard practice for hearing aid fittings, a system or apparatus configured to address this deficiency would fill an important niche for the more than 13,000 Audiologists currently practicing in the U.S. and the many more hearing professionals and hearing aid dispensers practicing worldwide.
There remains a need for a simple, efficient, and intuitive method by which hearing professionals (e.g., clinicians, audiologists, and technicians) can assess an operational state of a hearing device (including, hearing aids) incorporating non-linear frequency shifting algorithms.